Many resins, such as polymers comprising polyethylene, have historically been vulnerable to polymer extrudate distortion and surface roughness, notably “shark skin”, when subjected to high velocity through resin shaping devices. Such phenomena, referred to herein as exit region surface melt fracture, or “SMF”, are seen to occur in a variety of instances, including without limitation, dies for pelletizing, sheeting, or blown film. In conventional resin shaping devices such as die-based, underwater pelletizers, SMF can be the source of several downstream processing problems, such as polymer fines generation, pellet dryer fouling, poorly dewatered pellets, decreased pellet bulk density, poor bulk solids bin flow, physical segregation when mixed with other components in downstream processing, and non-uniform, defective surface, finished polymer parts.
The aforementioned phenomena affects some resins more than others. As an example, LLDPE resins are typically produced using either a titanium-based, Zielger-Natta (Z/N) catalyst or Group IVB-based (e.g., Zr or Ti) metallocene catalyst. It has been found that metallocene-catalyzed LLDPE (“mLLDPE”) typically will undergo SMF at shear stress levels roughly 20% lower than their Z-N-type counterparts. Generally, LLDPE resins (the term used herein to refer to both Z/N and metallocene-catalyzed resins unless otherwise specified) with a Melt Index less than or equal to about 1.0 dg/min (ASTM Method D-1238, Procedure B), are observed to be particularly susceptible to SMF.
Additives designed to be polymer processing aids (PPA) have been developed with the specific intent of reducing or eliminating the polymer SMF phenomenon, and it is conventional to add such additives as a matter of course. For instance, U.S. Pat. No. 6,187,397 suggests the use of fluorinated elastomers as processing aids “in the usual proportions . . . generally of the order of 500 ppm”. See also, for instance, U.S. Pat. Nos. 6,552,129; 6,017,991; and 5,089,200.
PPAs have proven somewhat useful to the secondary manufacturer, e.g., the film processor. However, the primary polymer manufacturer, i.e., the polymer processor, does not realize the same benefits PPA reduction of additive-induced SMF inhibition when processing the original polymer into pellet form in the conventional pelletizer. The incorporation of PPAs adds to the manufacturing cost of the product and may not be acceptable in the final product. Furthermore, the additive approach is ineffective in addressing the SMF phenomenon under commercial, high flow rate processing conditions, especially for mLLDPE resins.
Micro-scale roughness of the die capillary or die exit hole, die capillary material selection, and capillary geometry have been postulated as causes of SMF. These effects are called “microscale roughness effect” and may account for some of the variation and conflicting results reported in the open literature. While minor improvements may be achieved for those dies that have substantial defects by, for instance, polishing of the die orifice or capillaries, such improvements cannot substantially improve or eliminate SMF.
U.S. Pat. No. 6,474,969 B1 discloses a die and die assembly for use in association with an underwater pelletizer. The die has a coiled heating element described by the patentee as being upstream from (i.e., distal) the die exit hole (see FIG. 1 of the reference) which, according to the patent, reduces or eliminates polymer solidification within the die.
The present inventors have surprisingly discovered that heating the die hole wall near to or at (i.e., proximate) the exit surface reduces or eliminates exit region surface melt fracture in polymers extruded through a shaping device.